Field
The present invention relates to an anode active material for a lithium secondary battery and a lithium secondary battery having the same. More particularly, the present invention relates to an anode active material for a lithium secondary battery including a silicon-based alloy with a small crystal size, and a lithium secondary battery having the same.
Description of Related Art
Different types of electrolytes are used for electrochemical devices widely used these days, for example, lithium secondary batteries, electrolytic condensers, electric double-layer capacitors and electrochromic display devices, as well as dye-sensitized solar cells of which various studies are being undertaken for future commercialization, and so the importance of electrolytes is increasing day by day.
In particular, lithium secondary batteries are attracting the most attention for its high energy density and long cycle life. Generally, a lithium secondary battery includes an anode made from carbon material or lithium metal alloy, a cathode made from lithium metal oxide, and an electrolyte made by dissolving a lithium salt in an organic solvent.
Initially, lithium metal was used as an anode active material for an anode of a lithium secondary battery. However, because lithium has low reversibility and low safety, currently carbon material is mainly used as an anode active material of a lithium secondary battery. The carbon material has low capacity compared with lithium, but is advantageous in that it has a small change in volume, excellent reversibility, and low price.
As the use of lithium secondary batteries are increasing, so does the demand for high-capacity lithium secondary batteries. Accordingly, there is a demand for high-capacity anode active materials that may substitute the carbon material having low capacity. In order to meet the demand, attempts were made to use metals as an anode active material, for example, Si, Sn, and the like, that have a higher charge/discharge capacity than the carbon material and that allow electrochemical alloying with lithium.
However, this metal-based anode active material has a great change in volume during charging/discharging, which may cause cracks to the active material. Secondary batteries using this metal-based anode active material may suddenly deteriorate in capacity and reduce in cycle life over repeated cycles of charging/discharging, and thus, are not suitable for commercial use.
To solve this problem, attempts have been made to use an alloy of Si and other metal or an alloy of Sn and other metal as an anode active material. However, the use of such an alloy contributes to the improvement of cycle life characteristics and prevention of volume expansion to some extent when compared with the use of metal alone as an anode active material, but the extent is insufficient for commercial use.
More specifically, an anode active material made from an alloy of Sn and other metal or an alloy of Sn and other metal has an Si or Sn phase capable of bonding with lithium and an irreversible phase incapable of bonding with lithium. In this instance, ideally, the Si or Sn phase capable of bonding with lithium and the irreversible phase incapable of bonding with lithium form particles of a nano-scale size and uniformly disperse the particles. To realize the ideal, an amorphous phase is preferably formed when manufacturing a silicon alloy, or even though a crystal phase is present, the size of the crystal phase needs to be at several nano scale. However, since the formation of a crystal phase is thermodynamically stable during cooling subsequent to melting in the manufacturing of an alloy, a crystal phase having a size of several microns is generally formed.
A crystal phase having a size of several microns still causes a change in volume during charging/discharging.